PostgreSQL offers
asynchronous notification via the LISTEN
and NOTIFY commands. A client session
registers its interest in a particular notification channel with
the LISTEN command (and can stop
listening with the UNLISTEN command).
All sessions listening on a particular channel will be notified
asynchronously when a NOTIFY command
with that channel name is executed by any session. A "payload" string can be passed to communicate
additional data to the listeners.

The function PQnotifies returns
the next notification from a list of unhandled notification
messages received from the server. It returns a null pointer if
there are no pending notifications. Once a notification is
returned from PQnotifies, it is
considered handled and will be removed from the list of
notifications.

After processing a PGnotify object
returned by PQnotifies, be sure to
free it with PQfreemem. It is
sufficient to free the PGnotify
pointer; the relname and extra fields do not represent separate
allocations. (The names of these fields are historical; in
particular, channel names need not have anything to do with
relation names.)

Example 31-2
gives a sample program that illustrates the use of asynchronous
notification.

PQnotifies does not actually
read data from the server; it just returns messages previously
absorbed by another libpq
function. In prior releases of libpq, the only way to ensure timely receipt
of NOTIFY messages was to constantly
submit commands, even empty ones, and then check PQnotifies after each PQexec. While this still works, it is
deprecated as a waste of processing power.

A better way to check for NOTIFY
messages when you have no useful commands to execute is to call
PQconsumeInput, then check
PQnotifies. You can use
select() to wait for data to arrive
from the server, thereby using no CPU power unless there is something to do.
(See PQsocket to obtain the file
descriptor number to use with select().) Note that this will work OK whether
you submit commands with PQsendQuery/PQgetResult or simply use PQexec. You should, however, remember to check
PQnotifies after each PQgetResult or PQexec, to see if any notifications came in
during the processing of the command.